Transformer Inrush Specification

What? A manufacturer providing deliberately misleading information in order to hide some ugly truth? I'm shoked!

Ask manufacturer the standard(BS/IEC)to which it is made.

What type of circuit breaker are you using? 4KVA is nothing, inrush is everything on power up on a breaker, all depends on what type of breaker you are using, vs. a switch to energize. Breakers are not meant to be a a good on /off switch.

As I said above, the circuit breaker is a 16A D-Curve Miniature Circuit Breaker.

If you are from the US you may not have met these before, they are an IEC thermal-magnetic circuit breaker. They are used in everything from domestic switchboards to industrial and commercial applications. A D-Curve should trip magnetically at >10 x In.

The breaker is not being used as an on/off switch, I don't know where that idea came from. It is the primary protection for the transformer.

Check rating of breaker to comply with Peak inrush current . check with supplier referring IEC standard of inrush current. Also check core material, type/routine test certificate of similar rating by authorise test house.

All the above is just an observation the entire application shall be a full responsibility of user.

Well, there you have it. If the breaker should trip at >10 x In, then it's on the border of coordination. 4000VA (4kVA) @ 400V is 10A full load (FLA). Inrush on a small transformer can easily be 20x or more of FLA. 147A was probably the minimum the manufacturer measured, but the inrush won't be exactly the same every time. 230A may be the max inrush you could experience, and so you need to increase your breaker rating.

Per NEC (and I'm not sure what codes apply in OP's location), a 600V or less transformer with both primary and secondary protection can have the primary protection rated at up to 250% of FLA (25 Amps).

A <...16A D-Curve Miniature Circuit Breaker...> will withstand 20 x 16A = 320A briefly without caring tuppence. The breaker is there to protect the wiring, not the <...Transformer....>.

What is the question again?

Another solution might be to put a line reactor in line with the transformer. It is nothing more than an inductor in series with the primary. They are not very expensive and they are great at suppressing the peak current at the moment when the switch occurs. You could also look for a "zero cross over" switch to make sure that you always start out with a sinusoidal feed. Just throwing the switch without respect to the instantaneous magnitude of voltage is what causes the occasional trip of the breaker. Sometimes you get it just right and at other times you hit it at or near the peak in the AC cycle. That is what causes your "random" trips.

Similar to starting of induction motors,if you connect a shunt capacitor of suitable rating at the moment of switching across the transformer,inrush current could be reduced without resonance.

The electrical and physical characterisitics of the materials, (magnet wires, core laminations, insulators,etc.), used in the manufacturing process coupled in the design ultimately determines this Inrush current. It's mostly dependent on the final or net reactances of the device. The manufacturer normally defines and have control over their respective product specifications, limited only by the quality of the raw materials and suppliers!

One of the options available to you is to change transformer supplier or switch to another brand of transformers, manufactured by different company.

Another option is for you to order directly from the manufacturer based or tailored according to your own specifications.

GMP, beside any good engineering practices used by the local engineers, is the only standards that you may call for the manufacturers that may adhere to...

I fear that I have not asked my question clearly enough, since most of the posts so far are simply telling me what I already know.

To put the question more clearly : Is transformer inrush current usually specified as a peak value, or is it normal for it to be given as an"effective" figure?

I ask this because my understanding was that it should be given as a peak figure, and have now encountered a variation to this which hasd created a problem.

FWIW, we will most likely solve this problem with an up-specced breaker which is more suited to a high-inrush load. We have laso looked into using NTCs, this works great but is less easy to retrofit to an established design than an upgraded breaker.

Cheers,

Paulusgnome

The manufacturer normally defines and have control over their respective product specifications, Unless you developed your own specifications and placed the order based on those directly from the manufacturer.
The above scenario was how we operate in the early 60's when I was still involved in the manufacturing of industrial and consumer electronics. Maybe things have changed?

It quite difficult to be accurate with the inrush current. At a worst case this can be 30 times the normal current but it is over within a split second. Of the 3 phases only one phase will be subject to the massive current and this very much depends on the state of the core when the transformer was powered down. Looking at a breaker curve the most suitable would be either a type C or D - the D being the better. A different manufacturer or feeding from a power system with a lower Ze would make the situation more onerous.

A manufacturer such as Merlin Gerin give recommendations for MCB sizes in the catalogue as I'm sure many otherS DO.

The inrush current of the transformer can not be exactly calculated as it depends on many factors. Generally the transformer inrush current may be 10 to 15 times of the transformer rated current. It may be even more than 15 times. The inrush current of the transformer depends on the following factors.

• The instant of charging of transformer- at zero crossing point of the waveform or at 90 degree.
• The number of primary turns- if transformer has on load tap changer, the maximum number of turns must be put into circuit by keeping the tap position at the minimum.

The inrush current of the transformer also depends on the number of transformer installed in the same circuit.

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Transformer inrush current